1. Field of the Invention
This invention relates to ballistic resistant materials having improved strength and lighter weights. More particularly, the invention pertains to improved ballistic resistant fabrics produced from drawn, high modulus fibers having a reduced fiber diameter and improved physical strength properties, without requiring modification of other properties such as fiber chemistry, binder resin type and binder resin content.
2. Description of the Related Art
Ballistic resistant articles containing high strength fibers that have excellent properties against projectiles are well known. Articles such as bullet resistant vests, helmets, vehicle panels and structural members of military equipment are typically made from fabrics comprising high strength fibers. High strength fibers conventionally used include polyethylene fibers, aramid fibers such as poly(phenylenediamine terephthalamide), graphite fibers, nylon fibers, glass fibers and the like. For many applications, such as vests or parts of vests, the fibers may be used in a woven or knitted fabric. For other applications, the fibers may be encapsulated or embedded in a polymeric binder material to form woven or non-woven rigid or flexible fabrics.
There are a number of parameters that affect the performance of a ballistic resistant material, and various ballistic resistant constructions are known that are useful for the formation of hard or soft armor articles such as helmets, panels and vests. For example, U.S. Pat. Nos. 4,403,012, 4,457,985, 4,613,535, 4,623,574, 4,650,710, 4,737,402, 4,748,064, 5,552,208, 5,587,230, 6,642,159, 6,841,492, 6,846,758, all of which are incorporated herein by reference to the extent not incompatible herewith, describe ballistic resistant composites which include high strength fibers made from materials such as extended chain ultra-high molecular weight polyethylene. These composites display varying degrees of resistance to penetration by high speed impact from projectiles such as bullets, shells, shrapnel and the like.
Particular parameters that affect the level of ballistic protection and the effectiveness of the ballistic resistant material include fiber construction, fiber surface, binder resin and binder resin content. However, fibers are the backbone of a ballistic material have the most significant impact on the effectiveness of a fabric in defending against penetration by fragments and bullets. High molecular weight, high modulus polyethylene filaments and fibers are particularly desirable materials to form such composite structures because they have very high strength to weight performance. They have a sufficiently high tensile modulus and tenacity to offer superior ballistic protection to a user, and sufficiently low weight to produce materials that are desirable to wear.
Many different techniques are known for the fabrication of high tenacity polyethylene filaments and fibers. Typically, such high tenacity polyethylene fibers are made by a spinning a solution containing ultra-high molecular weight polyethylene (UHMWPE) swelled with a suitable solvent into filaments of ultrahigh molecular weight polyethylene, followed by cooling the solution filaments to a gel state, then removing the spinning solvent. One or more of the solution filaments, the gel filaments and the solvent-free filaments are stretched or drawn to a highly oriented state in one or more stages. In general, such filaments are known as “gel-spun” polyethylene filaments. The gel spinning process discourages the formation of folded chain lamellae and favors formation of “extended chain” structures that more efficiently transmit tensile loads. Gel-spun filaments also tend to have melting points higher than the melting point of the polymer from which they were formed. For example, high molecular weight polyethylene of about 150,000, about one million and about two million molecular weight generally have melting points in the bulk of 138° C. Highly oriented polyethylene filaments made of these materials have melting points of from about 7° C. to about 13° C. higher. This slight increase in melting point reflects the crystalline perfection and higher crystalline orientation of the filaments as compared to the bulk polymer.
Various methods for forming gel-spun polyethylene filaments have been described, for example, in U.S. Pat. Nos. 4,413,110; 4,430,383; 4,436,689; 4,536,536; 4,545,950; 4,551,296; 4,612,148; 4,617,233; 4,663,101; 5,032,338; 5,246,657; 5,286,435; 5,342,567; 5,578,374; 5,736,244; 5,741,451; 5,958,582; 5,972,498; 6,448,359; 6,746,975; 6,969,553; 7,078,099 and 7,344,668, all of which are incorporated herein by reference to the extent not incompatible herewith. For example, U.S. Pat. Nos. 4,413,110, 4,663,101 and 5,736,244 describe the formation polyethylene gel precursors and the stretching of low porosity xerogels obtained therefrom to form high tenacity, high modulus fibers. U.S. Pat. Nos. 5,578,374 and 5,741,451 describe post-stretching a polyethylene fiber which has already been oriented by drawing at a particular temperature and draw rate. U.S. Pat. No. 6,746,975 describes high tenacity, high modulus multifilament yarns formed from polyethylene solutions via extrusion through a multi-orifice spinneret into a cross-flow gas stream to form a fluid product. The fluid product is gelled, stretched and formed into a xerogel. The xerogels is then subjected to a dual stage stretch to form the desired multifilament yarns.
U.S. Pat. No. 7,078,099 describes drawn, gel-spun multifilament polyethylene yarns having increased perfection of molecular structure. The yarns are produced by an improved gel spinning process and are drawn under specialized conditions to achieve multifilament yarns having a high degree of molecular and crystalline order. U.S. Pat. No. 7,344,668 describes a process for drawing essentially diluent-free gel-spun polyethylene multifilament yarns in a forced convection air oven and the drawn yarns produced thereby. The process conditions of draw ratio, stretch rate, residence time, oven length and feed speed are selected in specific relation to one another so as to achieve enhanced efficiency and productivity.
However, with the constantly broadening scope of ballistic threats, and the rapid expansion of people that feel a need to protect themselves from such threats, there is an ongoing need in the art to reduce the weight of ballistic resistant materials without reducing the effectiveness of the materials in defending against ballistic threats. The invention provides a solution to this need in the art, incorporating a plurality of high strength, low denier per filament monofilament fibers, a plurality of high strength, low denier multifilament fibers, or a combination of a plurality of high strength, low denier monofilament fibers and a plurality of high strength, low denier multifilament fibers in a specialized fabric construction to form thin and fine fiber layers and fabrics having excellent fiber spreading, enhanced strength and suitable fiber areal density without altering the overall fabric weight. These improved fabrics provide the end user the choice of selecting fabrics having improved ballistic performance with no increase in fabric weight, or reduced fabric weight without a corresponding reduction in ballistic performance. Each of the patents discussed herein represent advances in the state of the art, but none satisfy the needs met by the present invention.